If you have already ordered the Sigma then 'the Die is cast' etc.
As glsammy said, you probably won't get any improvement although you may find this lens a bit more useful as a general purpose lens.
Some people seem to struggle with the 150-500 but on a 40D I am frequently surprised by the results; for example two days ago and handheld, although I was resting my hands on some railings. 500 mm.
The light wasn't good, although that can sometimes be an advantage with egrets.
I suspect that a lot of negative reports about the 150-500 come from people who are expecting too much under difficult conditions. I find that it works best in the F8 to F14 range and F11 is ideal; which means that if you want to keep the ISO at 800 or preferably 400 with a sensible shutter speed you will be somewhat limited with regard to acceptable light conditions.
The OS works well and I have taken some quite sharp images while hand holding, but with a realistic shutter speed. Even on a good tripod, I like to keep the shutter speed at 1/200 or higher.
I suspect that if you are having problems with smaller birds the problem may be more to do with your settings. I always just use the centre auto focusing point as multiple points can give false focus by locking on to a closer twig instead of the bird. Even with only one focusing point I sometimes find that manual focusing works best.
The same goes for exposure; where spot metering is often more accurate.
And with regard to camera modes; forget the full auto programme and go for something more adjustable like Tv or Av, depending on the circumstances.
The Sigma will take a converter, but not the Canon converter, however I don't really recommend it for common use. It will get you a snap of a distant bird which will be perfectly acceptable for identification but don't expect to win competitions with it.
But, used with care, I feel sure that you will get good results from a 150-500.